In a Long Term Evolution (LTE) system, a random access channel (RACH) procedure of user equipment (UE) is as follows.
A higher-layer request triggers a physical layer to send a preamble. The request includes a preamble index, a target preamble received power (PREAMBLE_RECEIVED_TARGET_POWER), a corresponding random access-radio network temporary identifier (RA-RNTI), and physical random access channel (PRACH) resource information.
A preamble transmit power PPRACH is determined according to the following formula: PPRACH=min{PCMAX,c(i), PREAMBLE_RECEIVED_TARGET_POWER+PLc}_[dBm], where PCMAX,c(i) is a maximum permissible transmit power configured at the ith subframe of a serving cell c in which the UE is located, PLc is an estimation value obtained by the UE by estimating a downlink path loss of the serving cell c, and min{ } is a MIN operation.
Then, a preamble sequence is selected from a preamble sequence set using the preamble index. On an indicated PRACH resource, the selected preamble sequence is sent according to the preamble transmit power PPRACH.
There may be three cases after the UE sends the preamble sequence. In a first case, the UE detects a physical downlink control channel (PDCCH) according to the RA-RNTI, and detects a response to the sent preamble sequence in a corresponding downlink data block; then, the UE sends an uplink transport block according to content of the response. In a second case, the UE detects a PDCCH according to the RA-RNTI, but fails to find a response to the sent preamble sequence in a corresponding downlink data block; then, the UE determines, according to a higher-layer indication, whether to send a preamble sequence again. In a third case, the UE fails to receive a response from a network side; then, the UE determines, according to a higher-layer indication, whether to send a preamble sequence again.
In the second or third case, a Media Access Control (MAC) layer updates the target preamble received power and a new PRACH parameter, and sends a preamble sequence again. An updated target preamble received power is: preambleInitialReceivedTargetPower+DELTA_PREAMBLE+PREAMBLE_TRANSMISSION_COUNTER−1)*powerRampingStep, where preambleInitialReceivedTargetPower is an initial target preamble received power, sourcing from broadcast information sent by the network side; powerRampingStep is a power adjustment step, also sourcing from the broadcast information sent by the network side; DELTA_PREAMBLE is a preamble sequence format offset value, and is a protocol-specified value; and PREAMBLE_TRANSMISSION_COUNTER is a preamble transmission counter value, and can be obtained according to a current counter value.
Then, the physical layer is instructed to send the preamble sequence using the PRACH, the corresponding RA-RNTI, the preamble index, and the updated target preamble received power.
In a Universal Mobile Telecommunications System (UMTS), a random access channel procedure of UE is as follows.
On an available RACH subchannel corresponding to a given access service class (ASC), a next available access timeslot is selected. A new signature is randomly selected from an available signature corresponding to the given ASC. A preamble retransmission counter value is set as Preamble Retrans Max. If a preamble initial power (Preamble_Initial_Power) is less than a preset minimum value, a preamble required power is set between the preamble initial power and the preset minimum value; otherwise, a preamble required power is set to the preamble initial power. If the preamble required power is greater than a preset maximum value, a preamble transmit power is set to the preset maximum value; if the preamble required power is less than a preset minimum value, a preamble transmit power is set between the preamble required power and the preset minimum value. A preamble sequence is sent using the selected uplink access timeslot, the signature, and the preamble transmit power. If the UE fails to detect an acquisition indication of the corresponding signature in a downlink access timeslot corresponding to the uplink access timeslot, a next available access timeslot is selected on an available RACH subchannel corresponding to the given ASC. A new signature is randomly selected from an available signature corresponding to the given ASC. The preamble required power is increased by one power adjustment step (Power Ramp Step [dB]). If a preamble required power exceeds a maximum permissible power 6 dB, the UE sends a status (“no positive response available on an acquisition indication channel”) of a physical layer to a MAC layer and exits a random access procedure at the physical layer. The preamble retransmission counter is decreased by 1. If a preamble retransmission counter is greater than 0, a preamble sequence is sent again using the selected uplink access timeslot, the new signature, and the preamble transmit power. If a preamble retransmission counter is equal to 0, the UE sends the status (“no positive response available on an acquisition indication channel”) of the physical layer to the MAC layer and exits the random access procedure at the physical layer. If a negative acquisition indication is detected, the UE sends a status (“no positive response available on an acquisition indication channel”) of a physical layer to a MAC layer and exits a random access procedure at the physical layer.
The UE calculates, based on measurement of a downlink signal power, an initial transmit power Preamble_Initial_Power of a PRACH as follows: Preamble_Initial_Power=CPICH_TX_Power−CPICH_RSCP+UL_interference+Constant Value, where CPICH_RSCP is a received pilot channel power obtained by the UE by means of measurement, CPICH_Tx_Power is a transmit power of a downlink pilot channel, UL_interference is an uplink interference, and Constant_value is a constant. CPICH_Tx_Power, UL_interference, and Constant_value may be sent by a network to the UE using a broadcast channel.
It can be seen that, in the prior art, a manner of improving a transmission success rate by increasing a transmit power has a limited effect.